In the field of the electrophotographic type image forming apparatus, the structure is known in which elements such as a photosensitive drum and a developing roller as rotatable members contributable for image formation are unified as a cartridge which is detachably mountable to a main assembly of the image forming apparatus (main assembly). Here, in order to rotate the photosensitive drum in the cartridge, it is desirable to transmit a driving force thereto from the main assembly. It is known, for this purpose, to transmit the driving force through engagement between a coupling member of the cartridge and a driving force transmitting portion such as a drive pin of the main assembly side of the apparatus.
In some types of image forming apparatuses, a cartridge is demountable in a predetermined direction substantial perpendicular to a rotational axis of the photosensitive drum. In a known main assembly, the drive pin of the main assembly is moved in the rotational axis direction by an opening and closing operation of a cover of the main assembly. More particularly, a patent specification 1 discloses a structure in which a coupling member provided at an end portion of the photosensitive drum is pivotably relative to the rotational axis of the photosensitive drum. With this structure, the coupling member provided on the cartridge is engaged with the drive pin provided in the main assembly, by which the driving force is capable of being transmitted from the main assembly to the cartridge, as is known.    [Prior art reference] Japanese Laid-open Patent Application 2008-233867.